A feed pump of this type is known from European Patent Application EP 0 446 454 A1, for example. When such feed pumps are installed into a hydraulic system, the problem arises that the system, along with the feed pumps, must be extensively evacuated before being filled with a hydraulic pressure medium to assure the highest possible filling capacity with the hydraulic pressure medium, and thus total effectiveness of the system. Air that may have dissolved in the hydraulic pressure medium causes an elasticity in the pressure medium that is undesirable because of a resilience of the pressure medium column that conveys the pressure. Air bubbles or gas bubbles contained in the system are particularly disadvantageous. This applies especially to hydraulic brake systems, which only operate reliably when free from air bubbles.
With a known feed pump, when the pump is not in use, the valve-closing body is held fixedly and tightly on its valve seat by the valve spring. For instance, the valve spring is embodied such that more than 1 bar, and preferably approximately 6 bar, are required to open the outlet valve. This practically excludes the evacuation of the work cylinder of the pump. Post-evacuation filling of the system with a pressure medium coming from a reservoir of a main brake cylinder, for example, is not permitted with the use of a feed pump of this type.